1. Field of the Invention
The present invention relates to a frequency stabilization circuit connected between a feeder circuit and an antenna element, and an antenna device and a communication terminal apparatus that include the frequency stabilization circuit.
2. Description of the Related Art
In recent years, mobile communication terminal apparatuses, such as portable phones, may require compatibility with communication systems, such as a GSM (Global System for Mobile Communications), a DCS (Digital Communication System), PCS (Personal Communication Services), and a UMTS (Universal Mobile Telecommunications System), as well as a GPS (Global Positioning System), a wireless LAN, Bluetooth (registered trademark), and other systems. Thus, antennas for such communication terminal apparatuses are required to cover a plurality of frequency bands of 800 MHz to 2.4 GHz.
Generally, the antennas have a frequency characteristic in the impedance of each antenna, so that when an impedance matching circuit is inserted between a feeder circuit and the antenna, matching with the feeder circuit cannot be achieved over a wide frequency band. For example, FIG. 1 shows an example that represents, on a Smith chart, an impedance trace of a simple whip antenna. The correspondence between each marker in FIG. 1 and frequency is as follows:
m10: 824 MHz
m11: 960 MHz
m12: 1.71 GHz
m13: 1.99 GHz
The impedance of 824 MHz and 960 MHz frequency bands (low band) is approximately 10Ω and the impedance of 1.71 GHz and 1.99 GHz frequency bands (high band) is approximately 28Ω.
Thus, when the frequency range to be handled becomes a wide frequency band, the impedance of the antenna significantly varies according to the frequency, making it impossible to match, in a single matching circuit, the impedance of the antenna with the impedance (50Ω) of the feeder circuit over a wide frequency band.
Accordingly, as an antenna to cover a plurality of frequency bands, a tunable antenna is known. The tunable antenna includes a matching circuit including a variable capacitance element, as disclosed in Japanese Patent Laid-Open Publication No. 2000-124728 and Japanese Patent Laid-Open Publication No. 2008-035065.
FIG. 2 is a view showing a major portion of a mobile receiver equipped with a matching unit as shown in Japanese Patent Laid-Open Publication No. 2008-035065. The matching unit 1 is connected to an antenna element ANT and a control signal source 6 so as to define an antenna matching circuit 100. The antenna matching circuit 100 receives a channel selection command SEL and changes the capacitance of the matching unit 1 so as to provide matching of the circuit impedance viewed from a receiving circuit 8 in accordance with the reception frequency of a corresponding channel. Then, the antenna matching circuit 100 outputs a radio signal received by the antenna ANT to the receiving circuit 8. The receiving circuit 8 amplifies the radio signal received from the antenna matching circuit 100 and outputs the signal as a reception signal RF.
However, the tunable antennas including a variable capacitance element, in general, require a circuit for controlling the variable capacitance element, that is, a switching circuit for switching the frequency band, such that the circuit configuration is complicated. In addition, since loss and distortion in the switching circuit are relatively large, a sufficient gain is difficult to obtain. Furthermore, since tuning requires a certain amount of time, the tunable antennas cannot be used for a communication frequency band that must be switched in an instant.